Trapezoid signal generating circuit

ABSTRACT

A trapezoid signal generating circuit has a charging and discharging circuit for a capacitor to generate a trapezoid signal which has less change at its rising portion and falling portion. Current output circuits supply a charging current and a discharging current in accordance with a voltage outputted from a current control circuit, respectively. The current control circuit has a charging and discharging circuit similar to the charging and discharging circuit, and produces an output voltage. This voltage increases in accordance with a linear function for a period from a time point when an input signal changes its level to a time point when the voltage reaches a reference voltage, and decreases thereafter in accordance with a linear function. The current flowing into the capacitor also increases and decreases in accordance with the linear function, so that the terminal voltage of the capacitor increases and decreases in accordance with a quadratic function.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by referenceJapanese Patent Application No. 2003-90967 filed on Mar. 28, 2003.

FIELD OF THE INVENTION

The present invention relates to a trapezoid signal generating circuit,which generates a trapezoid signal by controlling a charging anddischarging current of a capacitor.

BACKGROUND OF THE INVENTION

When a level of a rectangular signal changes, noises will arise due toharmonic components included in the sharp rising portions and the sharpfalling portions of the rectangular signal. For instance, if therectangular signal having the frequently changing levels is used invehicle-mounted electronic devices, radio noises are generated. It iseffective to use a trapezoid signal in place of the rectangular signalfor reducing such noises.

JP-A-52-112263 discloses a waveform shaping circuit, which changes theinclination of one of the rising slope and the falling slope of atrapezoid signal in the middle of the slope. This circuit increases theinclination of the sharper one of the inclination-changed slopes to besharper than the inclination of the other slope, which is notinclination-changed. This circuit decreases the inclination of the lesssharp one of the inclination-changed slopes to be less sharp than theinclination of the other slope, which is not inclination-changed.Specifically, this circuit has a capacitor, and changes a charging anddischarging current of the capacitor in steps according to the terminalvoltage of the capacitor.

FIG. 6 shows an example of a trapezoid signal generating circuit, andFIG. 7 shows the waveforms of operation signals of the trapezoid signalgenerating circuit 1. When an input signal Sin becomes a high level H, aswitch 6 turns off via an inverter 5. A capacitor 2 is charged with anoutput current I1 of a constant current circuit 3. Thus, the outputvoltage Vo outputted via a buffer 7 linearly increases to a fixed level.When the input signal Sin becomes a low level L, the switch 6 turns on.The capacitor 2 is discharged with a current I1 which corresponds to thedifference between the output currents of the constant current circuits3 and 4. Thus, the output voltage Vo linearly decreases to zero.

In the voltage Vo (trapezoid signal) thus generated, the inclinationchanges in a stepwise manner at the shoulder portions of its risingportion and falling portion (increase/decrease starting portion orincrease/decrease ending portion). As a result, it is difficult tosufficiently reduce the harmonic components. Therefore, the above priorart proposes to reduce changes of the inclination at the shoulderportions by changing the charging and discharging current in steps. Incase of changing the charging and discharging current in steps inaccordance with the terminal voltage of the capacitor 2, however,comparators in the number corresponding to the number of change stepsare necessitated. The circuit size will be complicated more as thewaveform is smoothed more.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide atrapezoid signal generating circuit, which generates a graduallychanging trapezoid signal while minimizing a circuit size.

According to the present invention, a first and a second current outputcircuits for controlling a charging and a discharging of a capacitor areprovided. A discharging current of the second current output circuit isset larger than a charging current of the first current output circuit.When a waveform control signal is at a first level, the capacitor isdischarged with a difference current between the charging current andthe discharging current. The rate of falling of a terminal voltage ofthe capacitor, that is, a trapezoid signal, is determined in accordancewith the difference current. When the waveform control signal is at asecond level, the discharging current is stopped. Thus, the capacitor ischarged with the charging current of the first current output circuit.The rate of rising of the trapezoid signal at this time is determined inaccordance with the charging current.

The first and the second current output circuits are constructed tosupply the charging current and the discharging current in accordancewith command signals, respectively. A current control circuit isconstructed to be able to continuously output the command signal like,for instance, a charging and discharging circuit described below. Thus,the charging current continuously increases after the time point whenthe waveform control signal changes from the first level to the secondlevel, and continuously decreases after the time point when thetrapezoid signal reaches a fixed reference level. In addition, both thecharging current and the discharging current continuously increase afterthe time point when the waveform control signal changes from the secondlevel to the first level, and continuously decreases after the timepoint when the trapezoid signal reaches the reference level.

The trapezoid signal results from an integration of the charging anddischarging current. As a result, the inclination of the slope of thetrapezoid signal at the shoulder portion (increase/decrease startingportion or increase/decrease ending portion) is changed very graduallyby the above control, so that the harmonic components included in thetrapezoid signal may be reduced. That is, a continuous current controlis attained based on the command signal generated by the current controlcircuit, in stead of a stepwise current control based on a terminalvoltage of a capacitor. As a result, a plurality of comparators need notbe used, and noise generation can be reduced while minimizing thecircuit size as much as possible. Because the time point to change thecharging and discharging current from an increase to a decrease iscontrolled based on a comparison of the trapezoid signal with thereference voltage, the terminal voltage of the capacitor after therising and after the falling can be controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electric circuit diagram showing a trapezoid signalgenerating circuit according to the first embodiment of the presentinvention;

FIG. 2 is a waveform diagram showing signal waveforms appearing atvarious points in the first embodiment;

FIG. 3 is an electric circuit diagram showing a trapezoid signalgenerating circuit according to the second embodiment of the presentinvention;

FIG. 4 is a waveform diagram showing signal waveforms appearing atvarious points in the second embodiment;

FIG. 5 is a waveform diagram showing signal waveforms appearing atvarious points in the second embodiment in the case that an input signalis different;

FIG. 6 is an electric circuit diagram showing a trapezoid signalgenerating circuit according to a prior art; and

FIG. 7 is a waveform diagram showing signal waveforms appearing atvarious points in the prior art.

DETAILED DESCRIPTION OF THE INVENTION

(First Embodiment)

A trapezoid signal generating circuit 11 shown in FIG. 1 is built in acontrol IC (semiconductor integrated circuit device) used in an ECU(electronic control unit) for controlling a chassis system of a vehiclesuch as doors, mirrors, seats, wipers, meters or an air-conditioner.This control IC also includes, in addition to this trapezoid signalgenerating circuit, digital circuits like a CPU or memories havingvarious functions, and analog circuits having various functions.

The control IC has a function of receiving from a plurality of switchesrespective on/off conditions. Although not shown in the figure, oneterminal of each switch is connected in common through a resistor, andthe other terminal of each switch is grounded. A trapezoid signalgenerated by the trapezoid signal generating circuit 11 is appliedperiodically to the common terminal, so that the CPU responsivelyreceives the voltage level of the one terminal of each switch through aninput port. The trapezoid signal is applied periodically to the switchesto reduce the current consumption and radio noises.

The trapezoid signal generating circuit 11 is constructed with acharging and discharging circuit 12 and a current control circuit 13,and operable with a power supply voltage Vcc between power supply lines14 and 15. The trapezoid signal is generated as a voltage Vo betweenboth terminals of a capacitor 16. This voltage Vo is outputted through abuffer 17.

A current output circuit (first current output circuit) 18 is connectedto the power supply line 14 and one terminal of the capacitor 16. Aseries circuit (second current output circuit) 21, which comprises aswitch 19 constructed with a transistor and a current output circuit 20,is connected between the one terminal of the capacitor 16 and the powersupply line 15, that is, between the terminals of the capacitor 16. Thecurrent output circuits 18 and 20 are for respectively supplying thecharging current and the discharging current of the capacitor 16. Thecurrent output circuit 20 allows the current of two times (K=2) of thecurrent of the current output circuit 18.

The current output circuits 18 and 20 change the output currents inaccordance with control signals applied thereto, respectively, whilemaintaining the relation of the current ratio of two. A V/I conversioncircuit 22 receives a voltage (command signal) V1 outputted from thecurrent control circuit 13, and applies control signals to the currentoutput circuits 18 and 20 in correspondence with this voltage.

The switch 19 switches its on/off condition in response to an inputsignal (waveform control signal) Sin applied thereto through an inverter23. Specifically, the switch 19 turns on in response to the L level(first level) of the input signal Sin, and turns off in response to theH level (second level) of the input signal Sin.

The current control circuit 13 is constructed with a charging anddischarging circuit 24, a comparator 25 and an exclusive-OR gate 26. Inthe similar manner as the charging and discharging circuit 12, thecharging and discharging circuit 24 is constructed with a capacitor(command signal capacitor) 27, a current output circuit 28 connectedbetween the power supply line 14 and one terminal of the capacitor 27, aswitch 20 and a current output circuit 30 connected in series betweenthe terminals of the capacitor 27, and a V/I conversion circuit 31. Acircuit portion of the charging and discharging circuit 24 other thanthe capacitor 27 operates as a charging and discharging current controlcircuit.

The comparator 25 is for comparing the voltage Vo with a referencevoltage Va, which is one half of the power supply voltage Vcc. Itproduces a signal Sa, which becomes a L level and a H level when thevoltage Vo becomes higher and lower than the reference voltage Va,respectively. The exclusive-OR gate 26 produces a signal Sb, which is anexclusive-OR logic of the input signal Sin and the signal Sa. The switch29 turns off when the signal Sb becomes the L level, and turns on whenthe signal Sa becomes the H level.

Next, the operation of the trapezoid signal generating circuit 11 isdescribed with reference to the input signal Sin, output signal Sa ofthe comparator 25, output signal Sb of the gate 26, input voltage Vin,output voltage V1 of the current control circuit 13, current I2 flowinginto the capacitor 27, current I1 flowing into the capacitor 16 andoutput voltage Vo of the trapezoid signal generating circuit 11 shown inFIG. 2. The input voltage Vin is a fixed level signal, and the inputsignal Sin is a periodic pulse signal having a fixed cycle period and afixed H level time width. In FIG. 2, the waveforms are illustrated withshortened cycle period for convenience of illustration. The input signalSin has in practice a cycle period of several tens of milliseconds and aH level time width of several hundreds of microseconds.

It is effective for reducing harmonic components at the rising portionand the falling portion of the voltage Vo, trapezoid signal, togradually change the voltage changing rate (inclination) at theincrease/decrease starting portions (A and C in FIG. 2) and at theincrease/decrease ending portions (B and D in FIG. 2) of the voltage Vo.Therefore, the increase/decrease starting portion and theincrease/decrease ending portion (shoulders) of the voltage Vo arechanged continuously and smoothly in accordance with not a linearfunction (straight line) but with a quadratic function (curve line).

The terminal voltage Vo of the capacitor 16 results from an integrationof the current I1 flowing into the capacitor 16. For this reason, thecurrent I1 may be controlled as shown in FIG. 2, so that it increases inaccordance with a linear function from the time point when the inputsignal Sin changes from the L level to the H level, and thereafterdecreases in accordance with a linear function. For this purpose, asshown in FIG. 2, the voltage V1 as the command signal is required torise in accordance with a linear function from the time point of thechange of the input signal Sin and thereafter fall in accordance with alinear function, if the voltage-current input and output characteristicof the V/I conversion circuit 22 and the current output circuits 18 and20 is linear.

More specifically, since the voltage Vo is decreased to about 0 volt andlower than the reference voltage Va at time t1 in FIG. 2, the outputsignal Sa of the comparator 25 is at the H level. When the input signalSin changes from the L level to the H level, the switch 19 turns off andthe output signal Sb of the gate 26 becomes the L level to turn off theswitch 29. As a result, a positive fixed current I2 flows from thecurrent output circuit 28 into the capacitor 27, and the terminalvoltage V1 of the capacitor 27 linearly rises from 0 volt. Thus, alinearly increasing current I1 flows from the current output circuit 18into the capacitor 16. Accordingly, the voltage Vo rises with agradually increasing inclination in accordance with the quadraticfunction.

When the voltage Vo exceeds the reference voltage Va at time point t2,the signal Sa changes from the H level to the L level and the signal Sbchanges from the L level to the H level. As a result, the switch 29turns on and a negative fixed current I2 flows into the capacitor 27 sothat the terminal voltage V1 of the capacitor 27 linearly falls. Thus, alinearly decreasing current I1 (>0) flows from the current outputcircuit 18 into the capacitor 16. Accordingly, the voltage Vo rises witha gradually decreasing inclination in accordance with the quadraticfunction. The voltage Vo stops rising when the voltage V1 becomes 0 attime point t3.

The output current ratio between the current output circuit 20 and thecurrent output circuit 18 and the output current ratio between thecurrent output circuit 30 and the current output circuit 28 are set totwo. In addition, the reference voltage Va is set to one half of Vcc.For this reason, the voltage Vo becomes generally equal to the powersupply voltage Vcc. Further, the waveforms of the voltage Vo between thetime points t1 and t2 and between the time points t2 and t3 becomessymmetric. This operation is similar to the operation when the voltageVo falls in the period from a time point t4 to a time point t6.

The control IC periodically outputs the voltage Vo in the trapezoidwaveform by using this trapezoid signal generating circuit 11, and isenabled to detect the on/off condition of each switch sequentially orconcurrently in the period (from time point t3 to time point t4) inwhich the voltage Vo is generally equal to the power supply voltage Vcc.

As described above, the current output circuits 18 and 20 of thecharging and discharging circuit 12 are constructed to allow thecharging current and the discharging current in accordance with thevoltage V1 outputted from the current control circuit 13, respectively,in this embodiment. The current control circuit 13 is constructed tooutput the voltage V1, which increases and decreases in accordance withthe linear function from the time point when the input signal Sinchanges its level, by the charging and discharging circuit 24 of asingle stage. As a result, the current I1 flowing into the capacitor 16increases and decreases in accordance with the linear function, and theterminal voltage (trapezoid signal) Vo of the capacitor 16 rises andfalls in accordance with the quadratic function. Thus, because thetrapezoid signal changes its slopes gradually at it shoulders inwaveform in particular, the harmonic components included in thetrapezoid signal are reduced. Accordingly, the radio noises which willaffect a radio receiver or other electronic devices mounted on a vehicleare reduced.

The trapezoid signal generating circuit 11 attains a continuous currentcontrol based on the voltage (command signal) V1 generated by thecurrent control circuit 13. This is different from a stepwise currentcontrol, which may be attained based on the terminal voltage Vo of thecapacitor 16. As a result, a smooth trapezoid signal can be generatedwithout requiring a plurality of comparators. Thus, the circuit size isreduced and the cost of IC is reduced.

In addition, the current control circuit 13 compares the voltage Vo andthe reference voltage Va by the use of a single comparator 25, andcontrols the magnitude of the voltage V1 from an increase to a decreasewhen the voltage Vo reaches the reference voltage Va. For this reason,the voltage Vo can be controlled accurately after the rising and fallingof the trapezoid signal.

(Second Embodiment)

FIG. 3 shows a trapezoid signal generating circuit, in which the sameconstructions as in FIG. 1 are represented with the same referencenumerals. This trapezoid signal generating circuit 32 is different fromthe trapezoid signal generating circuit 11 shown in FIG. 1 in that acurrent control circuit 33 is constructed with cascade-connected twocharging and discharging circuits 24 and 34.

The charging and discharging circuit 34 of the second stage isconstructed with, in the similar manner as the charging and dischargingcircuit 24, a capacitor (command signal capacitor) 35, a current outputcircuit 36 connected between the power supply line 14 and one terminalof the capacitor 35, a switch 37 and a current output circuit 38connected in series between the terminals of the capacitor 35, and a V/Iconversion circuit 39. The switch 37 is constructed to beon/off-controlled by the signal Sb in the similar manner as the switch29 is. The current output circuit 38 is constructed to output a twotimes current of the current output circuit 36. The charging anddischarging circuit 34 other than the capacitor 35 corresponds to acharging and discharging circuit. This charging and discharging circuit34 is constructed to receive the voltage V1 from the charging anddischarging circuit 24 and output a voltage (command signal) V2 to thecharging and discharging circuit 12.

FIG. 4 shows the input signal Sin of a duty ratio of 50%, output signalSa of the comparator 25, output signal Sb of the gate 26, input voltageVin, output voltage V1 of the charging and discharging circuit 24 of thefirst stage, output voltage V2 of the charging and discharging circuit34 (current control circuit 33), current I2 flowing into the capacitor27, current I3 flowing into the capacitor 35, current I1 flowing intothe capacitor 16 and output voltage Vo of the trapezoid signalgenerating circuit 11.

In this FIG. 4, the voltage V1, which the first-stage charging anddischarging current circuit 24 in the current control circuit 33outputs, is the same as the voltage V1 shown in FIG. 2. During theperiod from the time point t1 to the time point t2 in which the signalSb is at the L level, a linearly increasing current I3 flows into thecapacitor 35 from the current output circuit 36, and the voltage V2rises in accordance with a quadratic function. Accordingly, a current I1which increases in accordance with the quadratic function flows into thecapacitor 16 from the current output circuit 18. Thus, the voltage Vorises with a gradually increasing inclination in accordance with a cubicfunction.

During a time period from the time point t2 to time point t3 in whichthe signal Sb is at the H level, a linearly decreasing negative currentI3 flows into the capacitor 35, and the terminal voltage V2 of thecapacitor 35 falls in accordance with a quadratic function. As a result,the current I1 (>0) which decreases in accordance with the quadraticfunction flows into the capacitor 16 by the current output circuit 18and 20. Thus, the voltage Vo rises with a gradually increasinginclination in accordance with the cubic function. When the voltage V2falls to 0 at a time point t3, the voltage Vo stop rising. Thisoperation is similar from a time point t4 to a time point t6, whichcorresponds to the falling portion of the voltage Vo.

In case that the duty ratio of the input signal Sin is only aboutseveral %, the signals at each circuit position results in a trapezoidsignal Vo having a shortened time period t3–t4 as shown in FIG. 5.

By constructing the current control circuit 33 with two charging anddischarging circuits 24 and 33, the voltage V2 which is the commandsignal to the charging and discharging circuit 12 varies in accordancewith the quadratic function, and the voltage Vo which is the trapezoidsignal increases and decreases with the cubic function. As a result, thetrapezoid signal is changed at a slower rate at its shoulders, and theharmonic components included in the trapezoid signal can be reducedmore.

(Other Embodiments)

The present invention is not limited to the above embodiments, but maybe modified as follows.

In the first embodiment, a certain offset voltage may be provided sothat the voltage V1 which is the command signal does not fall to 0 volt.With this offset voltage, an offset current continues to flow todischarge the capacitor 16 while the input signal Sin is at the L level,and the terminal voltage Vo of the capacitor 16 can be decreased to 0volt without fail. In addition, the offset current continues to flow tocharge the capacitor 16 while the input signal Sin is at the H level,and the terminal voltage Vo of the capacitor 16 can be increased to thepower supply voltage Vcc without fail. Therefore, even when thecapacitor 16 is repetitively charged and discharged, the capacitor 16 isprevented from producing an offset voltage between its terminals. Acertain offset voltage is preferably provided similarly in the secondembodiment, so that the voltage V2 does not fall to 0 volt.

The current control circuits 13 and 33 may be constructed with three ormore cascade-connected charging and discharging circuits, or constructedwith different circuits other than charging and discharging circuits.

The ratio of output currents of the current output circuit 20 and thecurrent output circuit 18 is not limited to 2. Generally, the currentoutput circuit 20 is only required to produce a current of K times (K>1)of the current of the current output circuit 18. This relation is alsoapplicable between the current output circuit 28 and the current outputcircuit 30 and between the current output circuit 36 and the currentoutput circuit 38.

The reference voltage Va is not limited to one half of the power supplyvoltage Vcc.

1. A trapezoid signal generating circuit comprising: a capacitor forproducing and outputting a trapezoid signal generating circuit trapezoidsignal; a first current output circuit connected to the capacitor forsupplying to the capacitor a charging current having a first magnitude;a second current output circuit connected to the capacitor for supplyingfrom the capacitor a discharging current having a second magnitude whichis K times (K>1) the charging current when a waveform control signalapplied to the second current output circuit is at a first level, andstopping the discharging current when the waveform control signal is ata second level; and a current control circuit for receiving a trapezoidsignal generating circuit input signal and being connected to the firstcurrent output circuit and the second current output circuit forsupplying the first current output circuit with a command signal basedon the input signal so that the charging current continuously increasesfrom a first time point when the waveform control signal changes fromthe first level to the second level and continuously decreases after thetrapezoid signal produced and output by the capacitor reaches apredetermined reference level, and for supplying the first and thesecond current output circuits with the command signal so that both ofthe charging current and the discharging current continuously increasefrom a second time point when the waveform control signal changes fromthe second level to the first level and continuously decrease after thetrapezoid signal reaches the predetermined reference level.
 2. Thetrapezoid signal generating circuit according to claim 1, wherein thecurrent control circuit generates the command signal so that thecharging current and the discharging current increase and decrease inaccordance with a linear function with respect to an elapse of time fromeach time point of a level change between the first level and the secondlevel in the waveform control signal.
 3. The trapezoid signal generatingcircuit according to claim 1, wherein the current control circuitgenerates the command signal so that the charging current and thedischarging current increase and decrease in accordance with a quadraticfunction with respect to an elapse of time from each time point of alevel change between the first level and the second level in thewaveform control signal.
 4. The trapezoid signal generating circuitaccording to claim 1, wherein the predetermined reference level is setto one half of a power supply voltage, and the current control circuitgenerates the command signal so that each of the charging current andthe discharging current increases and decreases at changing rates equalto each other.
 5. The trapezoid signal generating circuit according toclaim 1, wherein the current control circuit generates the commandsignal, which enables a continuous flow of a first predetermined offsetcurrent as the discharging current while the waveform control signal isat the first level, and enables a continuous flow of a secondpredetermined offset current as the charging current while the waveformcontrol signal is at the second level.
 6. The trapezoid signalgenerating circuit according to claim 1, wherein: the first and thesecond current output circuits respectively produce the charging currentand the discharging current in accordance with the command signal; thecurrent control circuit includes one or more cascade-connected chargingand discharging circuits and produces the command signal from a last oneof the one or more cascade-connected charging and discharging circuits;and the one or more cascade-connected charging and discharging circuitseach include a command signal capacitor which produces the commandsignal, and a charging and discharging control circuit which charges thecommand signal capacitor with a first current corresponding to an inputvoltage from each time point of a level change between the first leveland the second level of the waveform control signal and discharges thecommand signal capacitor with a second current corresponding to theinput voltage after the trapezoid signal reaches the predeterminedreference level.
 7. The trapezoid signal generating circuit according toclaim 6, wherein the current control circuit includes a single-stagecharging and discharging circuit, and a fixed voltage is applied to thesingle-stage charging and discharging circuit.
 8. The trapezoid signalgenerating circuit according to claim 6, wherein the current controlcircuit includes a two-stage charging and discharging circuit, and afixed voltage is applied to a first stage of the two-stage charging anddischarging circuit.
 9. A trapezoid signal generating circuitcomprising: a reference signal input for receiving an input signal; acurrent control circuit coupled to the input for receiving the inputsignal and for generating and outputting a command signal based on theinput signal; a first current output circuit coupled to the currentcontrol circuit for receiving the command signal from the currentcontrol circuit; a second current output circuit coupled to the currentcontrol circuit for receiving the command signal from the currentcontrol circuit; a capacitor coupled to both the first current outputcircuit and the second current output circuit for producing a trapezoidsignal based on currents supplied by the first current output circuitand the second current output circuit; and a circuit output foroutputting the trapezoid signal produced by the capacitor, wherein thefirst current output circuit is for supplying to the capacitor acharging current having a first magnitude, and the second current outputcircuit is for supplying from the capacitor a discharging current havinga second magnitude which is K times (K>1) the charging current when awaveform control signal applied to the second current output circuit isat a first level, and is for stopping the discharging current when thewaveform control signal is at a second level.
 10. The trapezoid signalgenerating circuit of claim 9, wherein the current control circuit isfor supplying the first current output circuit with the command signalso that the charging current continuously increases from a first timepoint when the waveform control signal changes from the first level tothe second level and continuously decreases after the trapezoid signalproduced by the capacitor reaches a predetermined reference level, andfor supplying the first and the second current output circuits with thecommand signal so that both of the charging current and the dischargingcurrent continuously increase from a second time point when the waveformcontrol signal changes from the second level to the first level andcontinuously decrease after the trapezoid signal reaches thepredetermined reference level.